The present invention relates to the field of semiconductor chip mounting and connection, and more particularly relates to methods of making anisotropic conductive elements for use in microelectronic packages.
Modern electronic devices utilize semiconductor components, commonly referred to as xe2x80x9cintegrated circuits,xe2x80x9d which incorporate numerous electronic elements. These chips are typically mounted on substrates that physically support the chips and electrically interconnect each chip with other elements of the circuit. The substrate may be part of a discrete chip package, such as a single chip module or a multi-chip module, or may be a circuit board. The chip module or circuit board is typically incorporated into a large circuit. An interconnection between the chip and the chip module is commonly referred to as a xe2x80x9cfirst levelxe2x80x9d assembly or chip interconnection. An interconnection between the chip module and a printed circuit board or card is commonly referred to as a xe2x80x9csecond levelxe2x80x9d interconnection.
The structures utilized to provide the first level connection between the chip and the substrate must accommodate all of the required electrical interconnections to the chip. The number of connections to external circuit elements, commonly referred to as xe2x80x9cinput-outputxe2x80x9d or xe2x80x9cI/Oxe2x80x9d connections, is determined by the structure and function of the chip.
The rapid evolution of the semiconductor art in recent years has created an intense demand for semiconductor chip packages having progressively greater numbers of contacts in a given amount of space. An individual chip may require hundreds or even thousands of contacts, all within the area of the front face of the chip. Certain complex semiconductor chips currently being used have contacts spaced apart from one another at center-to-center distances of 0.1 mm or less and, in some cases, 0.05 mm or less.
One method utilized to interconnect semiconductor chips having closely spaced contacts includes using anisotropic conductive material. In U.S. Pat. No. 5,627,405, issued May 6, 1997, Chillara discloses an integrated circuit assembly comprising an integrated circuit, a dielectric substrate and an anisotropic electrically conductive layer interposed between the dielectric substrate and the integrated circuit. The anisotropic electrically conductive layer is electrically conductive in directions that are parallel to an electrically conductive axis and is electrically insulative in other directions.
However, there continues to be a need for improvement in elements and methods of making anisotropic conductive elements for use in microelectronic packaging.
In one preferred embodiment of the present invention, a method of making an anisotropic conductive element for use in microelectronic packaging includes the step of providing a layer of an anisotropic conductive material incorporating a dielectric material, such as a polymeric material or a curable silicone elastomer. The layer of anisotropic conductive material preferably includes a plurality of electrically conductive particles in the dielectric material. The dielectric material is preferably provided in a fluid or uncured condition. The dielectric material may be made more fluid by heating the anisotropic conductive material so that the viscosity of the material is reduced whereby the electrically conductive particles are relatively more free to move throughout the layer. The layer of anisotropic conductive material preferably has a pair of oppositely-directed major faces, a vertical direction between the major faces and horizontal directions parallel to the major faces.
The electrically conductive particles may include metal, such as solid metal balls, or elongated metal particles such as particles having longitudinal axes. The electrically conductive particles may also include elements having non-conductive cores which are coated with a layer of a conductive material. The non-conductive cores may include epoxy or other polymers, glass or silicone. Preferably, the conductive layers are provided uniformly about the cores.
A field is preferably applied to the layer of anisotropic conductive material so as to alter the configuration of the electrically conductive particles. The applied field may include an electrical field, a magnetic field or a combined electrical and magnetic field applied to the layer. In one preferred embodiment, the applying a field step includes biasing said first and second major faces with a different electrical potentials on at least some regions of the major faces. The electrically conductive particles preferably have longitudinal axes whereby the application of the electrical field turns the axes of elongation of at least some of the elongated particles toward the vertical direction. As a result, at least some of the electrically conductive particles are positioned in a substantially vertical direction between the major faces. Where the vertical dimension or thickness of the layer is larger than the longitudinal dimension of the particles, application of the field may bring some of the particles to a generally end-to-end disposition. The application of the field may also move at least some of the particles in horizontal directions so as to form areas of high particle concentration interspersed with areas of low particle concentration. The effects of turning the axes of elongation of at least some of the elongated conductive particles toward the vertical direction and moving at least some of the conductive particles in horizontal directions may be combined to provide areas of high particle concentration which generally include vertically-arrayed conductive particles which are closely congregated with one another. The areas of high particle concentration facilitate the conduction of electrical signals through the layer, from one major face of the layer to the second oppositely directed major face of the layer.
After the field has been applied to the layer of anisotropic conductive material so as to alter the configuration of the particles, the dielectric material is set or cured so that the dielectric material transforms into a non-fluid condition whereby the electrically conductive particles are substantially secured or frozen in place. In other words, the conductive particles are relatively less mobile throughout the dielectric material after the dielectric material has been set. However, in other embodiments the application of heat to set the curable dielectric material may occur simultaneously with the application of a field, such as a magnetic field.
The application of the magnetic or electrical field to the layer of anisotropic conductive material programs the layer preferably provides an interposer which may be juxtaposed between microelectronic elements for electrically interconnecting the microelectronic elements. The electrical and/or magnetic field generally alters the configuration of the particles to provide one or more substantially vertically-directed conductive paths through the layer of anisotropic conductive material. Each vertically-directed conductive path preferably includes a plurality of the electrically conductive particles which have been drawn into areas of high concentration by the application of the field. The programmed layer may be stored between one or more storage liners, such as thin flexible sheets of plastic. The storage liners protect the layer from contamination. A release treatment, such as TEFLON, may be disposed between the storage liners and the major surfaces of the layer of anisotropic conductive materials so that the storage liners may be easily removed from the layer prior to assembly with one or more microelectronic elements.
The layer prepared in accordance with the methods described above may also be used to electrically interconnect microelectronic elements. For example, a first microelectronic element such as a semiconductor chip having a plurality of contacts on a front surface thereof may be juxtaposed and abutted against the first major surface of the layer of anisotropic conductive material, with the contacts on the semiconductor chip preferably aligned with the vertically-directed conductive paths which have been programmed into the layer. A second microelectronic element such as a printed circuit board having a plurality of electrical contacts on the top surface thereof is then juxtaposed with and abutted against the second major face of the layer of anisotropic conductive material. The electrical contacts on the printed circuit board are preferably aligned with the contacts on the semiconductor chip and with the vertically-directed conductive paths of the layer so that the contacts of the semiconductor chip are electrically interconnected with the contacts of the printed circuit board.
A layer of anisotropic conductive material is typically compressed in order to lower resistance and improve electrical conduction through the layer""s conductive paths. The compression step may be performed before or, preferably, after the first and second microelectronic elements described above have been abutted against the respective first and second major faces of the layer. The first and second microelectronic elements are preferably moved toward one another so as to compress the layer in the vertical direction to lower the resistance of the vertically-directed conductive paths.
In other preferred embodiments of the present invention, a method of making a microelectronic package includes providing a microelectronic element having a plurality of electrical contacts on a first surface thereof, and then providing a layer of anisotropic material over the first surface of the microelectronic element. The layer includes a dielectric material in a fluid condition and electrically conductive particles in the dielectric material. A field, such as an electrical field or a magnetic field is then applied through the contacts on the first surface of the microelectronic element to the anisotropic conductive material so as to alter the configuration of the electrically conductive particles. In certain embodiments, during the applying a field step, the electrically conductive particles having elongated axes are turned so that the elongated axes are directed in a substantially vertical direction running between the first and second major faces of the layer of anisotropic conductive material. The application of the field may also move at least some of the electrically conductive particles in horizontal directions, the horizontal directions being substantially parallel to the major faces of the layer of anisotropic conductive material. As a result, at least some of the electrically conductive particles move into areas which are in substantial alignment with the electrical contacts so as to form areas within the layer of high particle concentration interspersed with areas of low particle concentration. The vertically-oriented particles and/or areas of high particle concentration provide conductive paths which enable electrical signals to be transmitted therethrough. After the field has been applied to the layer, the dielectric material is preferably set or cured, such as by using heat, so that the dielectric material is in a non-fluid condition after the moving step. A second microelectronic element, having a plurality of electrical contacts, may be abutted against the second major face of the layer before or after the field applying step so that the contacts of the first and second microelectronic elements confront one another. The vertically-oriented particles and/or particles in the areas of high particle concentration provide an electrical path through the layer and electrically interconnect the contacts of the first and second microelectronic elements. If the second microelectronic element is provided before the field applying step, the field may be directed through the contacts at the second microelectronic element as well as the first element. In certain embodiments, a substrate such as a flexible dielectric sheet is provided over at least one of the oppositely-directed first and second major faces of the layer of a material incorporating a curable dielectric material. The flexible dielectric sheet preferably includes a polymeric material.
In an additional aspect of the present invention, a layer of an anisotropic conductive material includes a pair of oppositely-directed major faces, a vertical direction extending between the major faces, and horizontal directions transverse to the vertical direction. The layer of anisotropic conductive material includes a dielectric material and a plurality of conductive particles in the dielectric material whereby the conductive particles may be distributed non-uniformly in the horizontal directions so as to provide areas of high particle concentration interspersed with areas of low particle concentration. Alternatively or additionally, at least some of the conductive particles may be elongated, and at least some of the elongated particles may have their axes of elongation disposed in substantially the vertical direction. The particles generally abut one another so as to provide low-resistance electrical paths between the major faces. The resistance of the electrical paths may be further reduced by compressing the layer so as to more closely array the conductive particles concentrated in the high particle concentration areas. The conductive layer may be placed in storage by applying storage liners over the respective first and second major surfaces of the layer. The storage liners protect the layer from contamination while in storage. The storage liners are preferably removed so that the layer may be assembled between microelectronic elements. The vertically-oriented conductive particles and/or the conductive particles in the areas of high particle concentration provide a series of substantially vertical paths through the layer so that separate and distinct electrical signals may be transmitted through the layer, with each distinct signal being electrically isolated from neighboring signals.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of preferred embodiments as set forth below and when taken in conjunction with the accompanying drawings.